The movie opens with Bick Benedict (Rock Hudson), the head of a Texas ranch, traveling to Maryland to buy a stud horse, War Winds. While there, he meets and courts socialite Leslie (Elizabeth Taylor) who becomes his wife. They return to Texas to start their life together on the family ranch, Reata.

Luz (Mercedes McCambridge), Bick's sister, and Leslie do not get along. Jett Rink (James Dean), the ranch's handyman, is envious of the Benedict wealth and is smitten with Leslie.

Luz dies after War Winds bucks her off, and as part of her will, Jett is given a plot of land on the Benedict ranch. Bick tries to buy back the land, but Jett refuses. Jett fences off his piece of land and names the property Little Reata.

Leslie gives birth to twins (Jordan Benedict III and Judy) and later has another daughter Luz Jr. Bick's dream is for his son Jordy to one day take over the family ranch. Even at a young age, Jordy is afraid of horses and prefers playing with his toy doctor's kit. This causes tension between Leslie and Bick.

Jett discovers oil on his property and when he gets his first gusher, he barges onto the Benedict's property proclaiming in front of the entire family and most of the town who have gathered for the twins' birthday party, that he will be richer than the Benedicts.

Jett starts an oil drilling company that makes him enormously wealthy. Bick resists the lure of oil wealth, preferring to remain a rancher. After World War II breaks out, Jett convinces Bick to allow oil production on the ranch to help the war effort.

In the postwar years, tensions in the Benedict household revolve around how the parents want to bring up their children. Bick stubbornly insists that Jordy (Dennis Hopper) must succeed him and run the ranch, but Jordy wants to become a doctor. Leslie wants Judy (Fran Bennett) to attend finishing school in in Switzerland, but Judy loves the ranch and wants to stay in Texas for her education studying agriculture.

Bick realizes Jordy is going to become a doctor despite his wishes. He hopes that Judy's new husband Bob (Earl Holliman) will take over the ranch, but Judy wants to start her own place, a little ranch for her and her husband.

The Benedict/Rink rivalry comes to a head when the Benedicts find Luz Jr. (Carroll Baker) and Jett Rink have been dating. Bick takes Jett to a kitchen room to fight him over Judy and Jordy but realizes that Jett is a drunken old man, who only has money and leaves. The party ends when Jett, completely drunk, passes out on the table right before his big speech. Luz Jr. sees him afterwards and discovers he is a lonely, unhappy, mess who has a crush on her mother.

The movie protrays how the oil industry transformed Texas ranchers during the early 20th century.

This movie also has a major sub-plot of racism against the Mexican Americans in Texas. When the movie starts, Bick and Luz are racist towards the Mexicans who work on their ranch, which upsets Leslie greatly. She also learns that Jett is racist as well. In one scene, Jett drives Leslie through a little Mexican American village where a baby is very sick. Jett tells Leslie not to get involved. Leslie does and makes sure a doctor visits the baby and this upsets her husband greatly. Leslie later arranges for a Mexican American doctor to look after the people in the village.

Bick's racism comes to a head, when his son Jordy marries a Mexican American (Elsa Cardenas) and they have a son. Bick slowly begins to see the Mexican Americans as people and his equal. In a very telling scene at a diner, Bick gets into a fist fight with the diner's owner when he refuses service to a family of Mexican Americans. At the gala, Jordy first gets into a fight with Jett when Jett has order his hotel staff not to serve Mexican Americans and his wife is refused service at a beauty salon.

This movie is way ahead of its time telling the racial tension that existed in Texas during the 20th century. Also Bick's beating at the hands of the racist diner owner shows that good does not always triumph over bad.

When World War II breaks out, Judy's husband Bob and Angel (Sal Mineo) the son of a ranch hand, are both drafted. Bob returns home safe but Angel is killed in the war, a reminder that Mexican Americans fought and were killed in World War II.

A third more minor sub plot is women as equals. This is not as apparant as the racism sub plot. But is shown beautifully in one scene after a dinner party at the ranch. The men are sitting in one corner talking politics. The women are in another corner, knitting. Leslie attempts to join in on the men's conversation and is told it is not her place. Leslie gives a wonderful speech about women and their contributions to the world and their rights.

Giant won the Academy Award for Directing. It was also nominated for Best Actor in a Leading Role (both James Dean and Rock Hudson), Best Actress in a Supporting Role (Mercedes McCambridge), Best Art Direction-Set Direction, Best Costume Design, Best Film Editing, Best Musical Score, Best Picture and Best Writing.